Marine seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) under the seafloor. This profile does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. In other words, such an image of the subsurface is a necessary tool today for those drilling exploration wells for minimizing the potential of finding a dry well. Thus, providing a better image of the subsurface is an ongoing process.
Marine seismic surveying is designed to image the sub-surface of the earth over a specific area. The principle of the acquisition of the seismic data is to sample the targeted area by traversing programmed adjacent and parallel sail lines over the targeted area with a vessel towing a seismic spread composed of one or more source arrays and one or more receivers. To obtain reliable and good quality seismic image, the data has to be acquired continuously over the area. The quality of the distribution of the source and receivers positions during the acquisition is monitored by the analysis of the surface-derived seismic coverage on the bin grid.
For example, as shown in FIG. 1, a marine seismic data acquisition system 100 includes a survey vessel 102 towing a plurality of streamers 104 (one shown) that may extend over kilometers behind the vessel. One or more source arrays 106 (or simply “sources”) may also be towed by the survey vessel 102 or another survey vessel (not shown) for generating seismic waves 108. Conventionally, the source arrays 106 are placed in front of the streamers 104, considering a traveling direction of the survey vessel 102. The seismic waves 108 generated by the source arrays 106 propagate downward and penetrate the seafloor 110, eventually being reflected by a reflecting structure 112, 114, 116, 118 at an interface between different layers of the subsurface, back to the surface 119. The reflected seismic waves 120 propagate upward and are detected by detectors 122 provided on the streamers 104. This process is generally referred to as “shooting” a particular seafloor 110 area.
Each marine seismic acquisition is typically designed in advance of the survey vessel(s) actually traversing the area to be imaged. For example, the area to be imaged can be divided into a grid of bins that have a set size depending on the acquisition parameters, typically 12.5×6.25 meters (see FIG. 2). The coverage is computed by calculating the number of mid positions of the triggered seismic source and of the receivers within the streamers that are located within each bin. These so-called common mid-points (CMPs) are classified by source/receiver distances (offsets) and sorted in offset classes/categories (e.g., near offset, mid offset and far offset). The mid-points could be also classified by azimuth of the source receiver direction. A typical quality criterion for seismic coverage is the number of mid-points located within a bin (fold) for a given offset class. Such quality criteria will typically be part of the survey specifications established, e.g., by the customer of the survey in conjunction with the survey designer.
Assuming no streamer feathering in the acquisition, the survey time is mainly driven by the spread width and the size of the survey area; in this case, the seismic coverage is uniform and corresponds to nominal perfect fold. However in a marine environment, the movement of seismic sources and streamers is very dynamic, responding to vessel steering, and more unpredictably, the effects of ocean currents. Because of these variables, the efficiency of the acquisition is related to the ability of the system to position the seismic spread such that the mid-point positions calculated during the acquisition design fill each bin with the minimal required coverage while at the same time also minimizing the number of over-folded bins. This, in turn, will minimize the number of sail lines to be traversed by the seismic vessel.
Given these variables, during the acquisition any area where the bins are filled with less than the nominal fold is subject to an assessment as to whether an additional pass with the seismic spread is required or not. Typically, the size of the “holes” (i.e., area where the fold is less than nominal) and the amount of mid points into the bins compared to nominal fold (in percentage) are parameters which are used to decide whether an additional pass is needed. Such additional passes by the survey vessel(s) are typically referred to as “infill lines”.
Therefore the actual sail lines of the vessel, of the source(s) and of the associated spread are an important factor for optimization of the efficiency of the acquisition, for building a coverage without holes and without over filling the bins.
Several tools and strategies are available for assisting the vessel, the source and the spread of receivers to follow the pre-determined tracks associated with the binning grid described above with respect to FIG. 2 when guiding the survey vessel(s) along their sail lines. These tools and strategies include, for example, source pre-plot smoothing, current prediction to match feather, line selection schedule and timing, weather and current monitoring, dynamic source steering system (e.g., winch based, deflector based systems), receiver/streamer steering system, steering controller, vessel steering optimization, active deflectors/wing steering, etc.
Thus, the vessel steering system is a central contributor for achieving any strategy to properly position a point of interest (e.g., center of sources or source receiver mid-point) within the spread along a pre-determined track. An example of an optimized steering system 300 on-board a seismic vessel which can be used to steer the vessel along the predetermined tracks is depicted in FIG. 3. Therein, at block 302, a first input used in the navigation system 300 is the pre-planning information described above, e.g., the definition and selection of the predetermined tracks. Additional inputs include information provided by an integrated navigation system, e.g., information regarding the current position of the equipment towed by the survey vessel(s) relative to a point of interest (block 304) and environmental information such as current and/or wind information collected by environmental sensors (block 306). These inputs are provided to one or more controllers 308, e.g., processors, which take this data and output control information for steering the vessel(s), sources, streamers and/or deflectors, etc. This output control information is then sent to the actuator(s) that are responsible for manipulating the different elements to perform the steering, e.g., an autopilot 310, on-board actuators 312 and/or at-sea actuators 314. As used herein, the term “at least one steering element” refers to one or more of the elements 308, 310, 312 and 314.
Historically, steering the vessel(s) along the sail line while performing seismic surveying has been done manually on board the ship using the information given by binning software using the pre-calculated plots described above. For the navigator (i.e., a human operator), the usual process is to watch the binning software screen, spot holes in the coverage by scanning the colors, possibly anticipatively, and to adjust the vessel course accordingly. In practice, the steering offset (DTO) is constantly adjusted (visually) by the human operator. A first drawback of this manual technique is that steering the vessel to juxtapose the coverage of a current acquisition line with the coverage of the already-acquired adjacent line(s) implies that navigators must constantly adjust their steering commands by taking the information from the binning software for all the binning offset classes into account. A second drawback is that the optimization is done visually by an operator to try to achieve two different objectives, i.e., to avoid creating holes in the coverage which require infill passes and to also avoid overfilling the bins to optimize the number of passes required to cover the full survey area, which objectives are typically in conflict with one another:
Thus performing the vessel steering manually is suboptimal and very difficult to achieve even for an experienced operator. Yet another drawback of manual steering correction is excessive steering caused by dual objectives mentioned above. Such excessive steering often results in a sail line which is too dynamic/excessively complex, which makes that sail line difficult to mimic when a future 4D seismic survey is to be performed based on a current seismic survey.
Another, more recent, technique for steering vessels during seismic surveys is to maintain a null steering offset with regards to the pre-plot line in the anticipation of repeating the same acquisition for 4D purposes and reservoir monitoring over time. A main drawback of this more recent technique is that while setting a null steering offset indeed facilitates the 4D survey which may be performed later, it also may result, for instance in the case of varying sea current conditions, in poor global coverage of the area being surveyed such that additional sail lines are required to achieve the targeted coverage.
Accordingly, it would be desirable to provide systems and methods that enable automated variable steering of the vessel (or other elements of the seismic surveying system) to improve an accuracy of the subsurface's image and to improve the efficiency of the acquisition in terms of reducing infill lines.
Some attempts have been considered to address this problem. For example, in U.S. Patent Publication US 2015/0183502 A1 to Chene, F. and Boudon, S., and U.S. Patent Publication 2015/0185350 A1 to Chene F., various steering techniques for seismic vessels are described. Additionally, in U.S. Patent Publication WO2015071491 to Tonchia, H. and Moulinier, T., entitled “Device and Method for Steering Seismic Vessel”, the disclosure of which is incorporated here by reference describes generating steering commands for an autopilot of a vessel to achieve a desired track of a point of interest associated with the towed equipment (rather than the point of interest being located on the vessel itself) for situations where the autopilot on the vessel was designed to receive vessel track commands and not the towed equipment point of interest track commands.
What is further needed are methods and systems for identifying the point of interest associated with the towed equipment and its desired track based on, among other things, properly determined coverage boundaries in a dynamic manner which can be provided as part of the steering information during the seismic acquisition and which further reduces the need for infills.
Accordingly, it would further be desirable to provide systems and methods for automated (or at least partially automated) vessel (or other element) steering for seismic surveys which avoid the aforementioned problems.